For emerging applications such as wireless domestic networks, intelligent networks or similar type networks, the use of directive antennas, that is antennas able to focus the radiated power in a particular direction of the space are proving particularly attractive. However, the laws of physics impose a minimum size for antennas, this size being all the more significant as the antenna is more directive or as its operating frequency is low.
Up until now, the use of directive antennas has remained limited to applications operating at very high frequencies, often with fixed beams, and do not have size constraints such as those of radar applications or satellite applications. Thus, for these application types, antenna devices are known that generate multiple beams but are composed of numerous modules that are often complex and costly. Conversely, antenna devices called retro-directive antennas enable directive beams to be formed very simply in a privileged direction of the space. Retro-directive antenna networks are based on the fact that each antenna of the network receives the incident signal of a source with a characteristic path-length difference, that is to say a different phase. This phase difference is characteristic of the direction of the emitting source. In fact, so that the signal to be sent is emitted in the direction of the source, it suffices that the phase difference between each antenna at transmission is opposite to that in reception so as to anticipate the path-length difference on the return path.
Among retro-directive antennas, the most well known network is the network call the “Van-Atta” network which is described, notably, in the U.S. Pat. No. 2,908,002 of 6 Oct. 1959. As shown in FIG. 1, a Van-Atta type retro-directive network is constituted of a number of radiating elements 1a, 1b, 2a, 2b, 3a, 3b that are symmetric with respect to the central axis Oy of the network. The radiating elements are connected by pairs, the radiating element 1a being connected to the radiating element 1b, the radiating element 2a connected to the radiating element 2b, the radiating element 3a connected to the radiating element 3b, via transmission lines 1, 2, 3 having equal electrical lengths, the antennas being symmetrically opposed with respect to the central axis of the network. In this case, the phase difference induced by the transmission lines is thus the same on all the radiating elements and the phase difference between two consecutive radiating elements is the same in reception of the signal and in transmission of the signal retro-directed to the closest sign. The phase differences between the signals of radiating elements of the transmitting network are thus opposed to the phase differences between the signals of the radiating elements of the receiving network. A retro-directivity of the transmitted signal is thus obtained.
However, this method has a certain number of significant disadvantages. To obtain the retro-directivity of the signal, the front of the incident wave must be flat. In addition, the antenna network must be flat or more or less symmetric with respect to the network centre. As the front of the incident wave must be flat, it is necessary that the network of radiating elements is positioned in the field area far from the transmitter source. As a result, the applications of Van-Atta type networks have only been, up to now, satellite or radar type applications.
As a result of studies made on these types of retro-directive networks, the present invention proposes to use the principle of a network of radiating elements to produce a system of multi-beam antennas that can be used in wireless communications, notably in wireless domestic networks or in peer to peer type networks communicating via wireless links, more specifically, in the scope of MIMO (Multiple Input Multiple Output) systems but also in antenna systems with a single antenna associated with processing systems operating with directive antennas.